The hundreds of thousands of gallons of red slurry that air tankers are dropping on Colorado forests to shield mountain houses from wildfires has a downside: It is toxic. Laced with ammonia and nitrates, it has the potential to kill fish and taint water supplies.

Federal authorities say they’re implementing new rules prohibiting application of fire-retardant chemicals within 600 feet of waterways. Air tanker pilots and crew commanders now are required to carry maps that identify sensitive terrain — such as areas where greenback cutthroat trout and Pawnee montane skipper butterflies are monitored as sentinel species.

But if wildfires threaten people, federal officials say, it’s bombs away, and air tankers will drop chemicals as close as they need. All “misapplications” must be reported and reviewed.

“There are other chemicals associated with it, but ammonia and the nitrates are the ones we’re currently aware of that are the largest concern,” said Doug Laye, the U.S. Fish and Wildlife Service regional coordinator for endangered species, who is overseeing the new policy for fire retardants. “These can change the water chemistry drastically. That can have a very quick and often lethal effect.”

So far this fire season, air tankers called to suppress wildfires have been dropping the fire retardants (the mix is called LC95A) at a record pace. As of Friday, more than 401,450 gallons had been dropped on Colorado forests this year, including 320,553 gallons on the lightning-sparked High Park wildfire west of Fort Collins, according to Forest Service records.

As it burns along the Cache la Poudre River — designated a Wild and Scenic River by the Forest Service — the High Park fire could overlap habitat for several species of sensitive fish.

Massive fish kills have been documented after fires were put out in northwestern states. A legal challenge by Forest Service Employees for Environmental Ethics compelled an environmental impact study. Federal biologists concluded that fire retardants alone will not cause extinctions. But the lawsuit led to issuance of the rules this year to minimize harm.

Laye said pilots and ground crews working west of Fort Collins are complying with the rules.

“Many wildfires burn in remote, rugged areas, and the application of fire retardant can slow the spread of a fire until ground forces can reach the area and begin construction of a fireline,” Segin said. “You can put it on homes. It works better on the ground. It slows the advance of a fire so that ground crews can get in there.”

Firefighters reaching mountain houses also spray chemical foams and wrap smaller structures with “fire shelter” material designed to repel flames. If there are ponds nearby, water pumps are set up to douse flames.

The federal government push to make more air tankers available to suppress western wildfires is likely to mean more use of fire-retardant chemicals.

Federal data show that, since 2007, air tankers have dropped an average of 486,385 gallons of fire retardant a year while suppressing wildfires in Colorado, where drought and overly dense forests favor large wildfires.

The Forest Service has been using retardant since the 1950s. These chemicals don’t douse flames as water does. Instead, the retardants cool and coat fuels, depleting fire of oxygen and slowing combustion as retardant salts change how fuels burn. Air tankers target houses and power grids.

This week, fire commanders loaded fire retardants into helicopters, which normally drop up to 900 gallons of water. A layer of retardant can slow fire for about two hours.

However, it was helicopters dropping water on flare-ups this week that saved two homes in the thick of the High Park fire, Segin said. “Like a military operation, it’s a combination of ground and air power coming together at a single point to suppress the fire.”

The maps designating sensitive terrain around Colorado apparently prioritize habitat for greenback cutthroats and the butterflies that live in riparian corridors — two species deemed most vulnerable to the fire-retardant chemicals.

Federal biologists would welcome development of better chemical mixes that could be less harmful and also meet firefighters’ needs, Laye said. “You obviously could use water anytime you would like.”

Bruce Finley covers environment issues, the land air and water struggles shaping Colorado and the West. Finley grew up in Colorado, graduated from Stanford, then earned masters degrees in international relations as a Fulbright scholar in Britain and in journalism at Northwestern. He is also a lawyer and previously handled international news with on-site reporting in 40 countries.

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